1. Field of the Invention
This invention relates generally to the control of adaptive optics used in imaging systems.
2. Description of the Related Art
Adaptive optics (AO) elements such as deformable mirrors or liquid crystal spatial light modulators allow rapid and precise control over an optical wavefront by shifting the phase of the incoming beam of light passing through the optical system. Adaptive optics elements are currently employed to correct uncontrolled wavefront errors arising from turbulent media (as in telescopic imaging), or random media such (as in microscopy and imaging of the human retina).
Conventionally, the adaptive optics element imparts the conjugate of the phase error in the wavefront so as to cancel out the wavefront error in a process known as phase conjugation. For example, an incoming aberrated wavefront reflects off a deformable mirror. The mirror's shape is controlled by an array of actuators so that the mirror cancels the aberrations in the incoming wavefront. The reflected wavefront then has no wavefront error.
In a typical application of adaptive optics, the actuators adjust the shape of the variable phase element in order to minimize the RMS wavefront error. When the incoming wavefront error is small, the adaptive optics element can completely correct the incoming beam. All adaptive optics elements, however, have a limited range of operation. There is a physical limit to the amount and speed at which a deformable surface can be deformed. For example, one commercial deformable mirror device is limited to a height deviation between neighboring pistons of 2 to 3 microns. This restriction on motion presents several problems when trying to correct severely aberrated wavefronts. On the other hand, LC-SLM devices typically have slower response times and can correct only small phase errors. For all of these devices, the traditional control of AO systems based on minimizing the RMS wavefront error breaks down when encountering large wavefront errors that cannot be fully compensated by the AO element.
Thus, there is a need for AO controllers that can provide good correction, even when the AO element is not capable of compensating fully for wavefront errors.